Electrical contact material and method of making the same

ABSTRACT

A material for electric contacts based on silver-tin oxide is obtained by mixing a powder of silver or an alloy mainly containing silver with a powder consisting mainly of tin oxide and 0.01 to 10 wt. % (in relation to the quantity of tin oxide) of an additive consisting of one or more compounds containing silver, oxygen and a metal from sub-groups II to VI of the periodic system and/or antimony, bismuth, germanium, indium and gallium, compacting the mixture and sintering it. The tin oxide may be replaced by zinc oxide.

This is a national stage application of PCT/EP93/02511, filed Sep. 16,1993.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention comprises a material for electric contacts based onsilver-tin oxide

2. Description of the Prior Art

Contact materials based on silver-tin oxide have begun to replace thehitherto preferred silver-cadmium oxide materials because the former areenvironment friendlier and generally have a longer life span. Theirthermal behavior is, however, unsatisfactory, since tin oxide, whensubjected to continuous current, has a tendency to produce poorlyconductive slag layers on the contact surface, influenced by electricarc. In order to overcome this disadvantage, it is known thatadmixtures, added in powder form to the powder metallurgically producedmaterial, bring about a temperature reduction at the contact. Suitableadmixtures in this sense have been mentioned in the patent literature,especially tungsten- and molybdenum-oxide and molybdenum carbide(DE-A-29 33 338, DE-A-31 02 067, DE-A-32 32 627). Further suitableadmixtures such as bismuth- and germanium-oxides have been mentioned(DE-A-31 02 067 and DE-A-32 32 627). These admixtures help wet tin oxideparticles so that when the contact area melts locally under the actionof electric arc, tin oxide remains suspended in fine particles. Apartfrom this favorable thermal behavior under continuous current, thesesadmixtures have also unfavorable side effects. The already somewhatunsatisfactory plastic deformation behavior, i. e. brittleness, ofsilver-tin oxide contact materials, which can be improved, for example,by annealing the tin oxide powder (DE-A-29 52 128), is worsened by theseadmixtures, since they promote embrittlement. This is particularly truefor bismuth and molybdenum oxide. A further disadvantage, especially ofthe tungsten and molybdenum compounds, is the fact that they tend totransfer material, especially during switching operations underAC1-loads (DIN 57660, Part 102), resulting in accelerated burning offand therefore reduced life span.

According to the teaching of WO 89/09478, a contact material with lowwelding tendency and minimal contact temperature under continuouscurrent can be obtained by creating a structure containing regions withlittle or no metal oxides, alternating with regions containing themajority of metal oxides, finely dispersed. For this purpose a powderedcompound, among other means, is commercially available, containing thepredominant portion of tin oxides and the other oxides and/or carbidesas well as a portion of the silver. This compound powder is combinedwith the remaining silver powder, and with the smaller remaining portionof metal oxides, mixed, condensed, sintered and transformed. In thismanner a useful material is obtained but through a relatively costlyprocess. Mentioned metal oxides are tungsten, molybdenum, bismuth,vanadium and copper.

From the paper by Christine Bourda et al. "Properties and effects ofdoping agents in AGSNO2contact materials", published in Proc. 16th Int.Conference on Electrical Contacts 7.-Sep. 12, 1992 in Loughborough, itis known that many admixtures made up of oxides react with silver or tinoxide; for example it was found that in a contact material produced fromsilver powder, tin oxide powder and molybdenum oxide powder or antimonyoxide powder, at temperatures reached during electric arcing, silver andmolybdenum oxide can combine into silver molybdate Ag₂ MoO₄, and silverand antimony oxide can combine into silver antimonate AgSbO₃. As tothese admixtures, the bibliographical reference indicates that,according to results of tests, they do not influence the wetability oftin oxide and silver, so that they are not expected to improve thetemperature behavior of contacts under continuous current.

In the older but not pre-published German patent application P 42 19333.8, a material for electric contacts had already been proposed on thebasis of silver-tin oxide which is obtained through mixing of a powderof silver or of a principally silver containing alloy with a tin oxidepowder, whose powder particles are doped with up to 5 weight % of anoxide or carbide of molybdenum, tungsten, bismuth, antimony, germanium,vanadium, copper or indium, condensing the mixture, sintering andtransforming it. The doped tin oxide powder is a compound powder whichcan be obtained through mixing tin oxide powder with the powdered dopingagent, annealing the mixture, so that the doping agent diffuses into thetin oxide powder particles, and segregating the surplus of the dopingagent from the tin oxide powder. A further process for obtaining dopedtin oxide powder is made known in P 42 19 333.8. A solution of a salt oftin and of a salt of the metal or metals, whose oxides or carbides makeup the doping agent, is sprayed into a hot oxidizing atmosphere in whichthe salts are to be decomposed so that a fine-grained compound powderprecipitates whose particles contain tin oxide and the doping agent.

SUMMARY OF THE INVENTION

The purpose of the present invention is to create a material of the kinddescribed at the beginning which, through use of admixtures exhibits anequally favorable thermal behavior as the already known materials do,but which is more ductile and has a longer life time when subjected tothe AC1 switching load case. This task is solved by a material withcharacteristics described in claim 1. A particularly suitable processfor producing such a material is the subject of claim 9. Favorablefurther developments of the invention are subject of dependent claims.

In the powder metallurgical production of a contact material on thebasis of silver-tin oxide, the invention additionally utilizes a powdercontaining one or more chemical compounds of silver, oxygen and a metalfrom subgroups II to VI and/or antimony, bismuth, germanium, gallium andindium, especially silver-tungsten-oxygen compounds,silver-molybdenum-oxygen compounds, silver-antimony-oxygen compounds andsilver-germanium-oxygen compounds. Although silver-antimonate andsilver-molybdate belong to this class of compounds which are known toform in a silver-tin-oxide-molybdenum-oxide and/or asilver-tin-oxide-antimony-oxide material and to have no favorableinfluence on the wetability of tin oxide (see Christine Bourda's paperreferenced above), one has achieved with the contact material accordingto the invention, a significantly lower temperature increase on thecontacts under continuous current as compared with known contactmaterials with comparable weight composition. It is suspected that thereason for this can be found in the contact material not being producedin the usual manner by mixing and sintering silver powder, tin oxidepowder and additional metal oxide powders, but, by starting out with apowder which contains, instead of a pure metal oxide such as, e.g.,MoO₃, a compound of type silver-metal-oxygen such as Ag₂ MoO₄,especially when this compound is completely or partially combined withtin oxide powder particles, forming a compound powder in whose particlestin oxide and the silver-metal-oxygen compound are combined; thiscompound powder is then mixed with silver powder and sintered into acontact material. Significant advantages are achieved in the powdermetallurgical production of the contact material according to theinvention, by mixing silver powder with a powder consisting mainly oftin oxide and one or more compounds of type silver-oxygen-metal.Surprisingly, it turned out that with the contact material according tothe invention a certain lowering of the contact temperature under givenconditions could be achieved with a significantly smaller portion of thechosen admixtures than hitherto possible with known technology. Firstexperience with the contact materials according to the invention showsthat a certain reduction of the contact temperature can be achieved,according to the invention, with only 1/2 to 1/10 of the admixturequantity necessary to achieve the same temperature reduction withhitherto known technology. This is also true for the example ofmolybdenum oxide whose share can be drastically reduced when replaced bysilver-molybdate, especially when combined with tin oxide particles.

This also results in a less brittle, i.e. more ductile contact material.A further advantage is the fact that because of the lower share ofnon-conductive admixture, the electric resistance of the contactmaterial is additionally lowered contributing additionally to a lowercontact temperature.

A further advantage of the invention is the fact that because of thelower share of admixture, the life span of contact pieces made from thecontact material is increased, especially under AC1test conditions. Theutilization of the powder according to the invention yields a lowerburning off compared with conventional silver-tin oxide contactmaterials using pure metal oxide admixtures such as tungsten oxide,molybdenum oxide or bismuth oxide.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The tin oxide particles are preferably coated with a layer ofsilver-metal-oxygen compounds. They effectively promote wetting of tinoxide particles with the liquid phase generated by arc. A tin oxidepowder modified in this manner can be made advantageously by mixing tinoxide powder with powdered admixture and annealing the mixture so thattin oxide powder particles are wetted by the admixture whereby a part ofthe admixture diffuses into the surface of the tin oxide particles,possibly forming a mixed oxide there.

In order to safeguard sufficiently against fusing of contact pieces, asrequired of silver-tin oxide materials, the material contains 5 to 20weight %, preferably 8 to 14 weight % tin oxide, and, in order for tinoxide to remain in suspension when melted under the influence ofelectric arc, as desired, tin oxide should contain a minimum of 0.1weight % admixtures, but not more than 2.5 weight %, and best not morethan 1 weight %.

A particularly preferred admixture is silver-molybdate, because of itsvery favorable effect on thermal behavior.

Annealing the mixture of tin oxide and the selected admixture is bestcarried out under an oxygen containing atmosphere, preferably air, at atemperature of between 500 deg. C. and 800 deg. C., the best temperaturebeing just above the melting point of the admixture, so that theadmixture is liquefied and can wet the surfaces of the tin oxideparticles. The admixture is thus deposited only where its favorablewetting effect is desirable, and can therefore be utilized withoutwaste. Considering the small quantities with which it is used, the tinoxide particles do not stick together; but should this occur on rareoccasions, then the material can be ground down.

Tin oxide and admixture can be combined not only through annealing butalso through deposition of the admixture on tin oxide particles throughapplication of chemical and physical separation processes.

The teaching contained in this patent can be applied to contactmaterials based on silver with zinc oxides. In these materialsadmixtures have so far not been used in practice. One rather tries toreduce contact temperatures in switching equipment through designmeasures. Through application of an admixture in the zinc-oxide powderaccording to the invention, a lowering of the the contact temperature ispossible with this type of material also.

EXAMPLES Example 1

With 100 parts by weight tin oxide powder with particle size <7 mmaccording to FSSS (FSSS=Fisher Sub-Sieve Sizer) and 0.5 parts by weightdi-silver-mono-molybdate Ag₂ MoO₄ of similar or equal particle size, apowder mixture is produced by dry mixing. This powder mixture is placedin shallow ceramic dishes and annealed in air at 600 deg. C. for theduration of 1 hour, thereby wetting the tin oxide powder with Ag₂ MoO₄.12 parts by weight of the annealed mixture are mixed with 85 parts byweight silver powder with approximate particle size of 20 mm (FSSSvalue). The mixture is cold-isostatically pressed into a block under 200MPa pressure, and subsequently sintered in air at 700 deg. C. for 2hours. The sintered block is forward extruded into a 5 mm thick string.The string is then flattened through hot rolling thereby producing asolderable silver backing, then, through cold rolling, given the finalthickness. From this strip contact platelets can be formed, as required,through shearing, stamping or saw cutting.

Example 2

A mixture is produced by dry mixing using 100 parts by weight tin oxidepowder with particle size <7 mm (acc. to FSSS) and 1 part by weightsilver-tetra-tungstate Ag₈ W₄ O₁₆ of similar or equal particle size.This powder mixture is placed in flat ceramic dishes and annealed in airfor approximately 1 hour at 700 deg. C., thereby wetting the tin oxidepowder with AG₈ W₄ O₁₆. 10 parts by weight of the annealed mixture aremixed with 90 parts by weight silver powder with a particle size ofapprox. 20 mm (acc. to FSSS). The mixture is cold-isostatically pressedinto cylindrical blocks under 200 MPa pressure and sintered in air forthe duration of 2 hours at 700 deg. C.

The sintered block is encased in silver, inserted hot into a backwardextrusion press (DE-OS 34 26 240). This process yields flat strips whichhave a solderable and weldable silver surface on one side. The finaldesired thickness is obtained through cold rolling. From this bandcontact platelet can be made, as required, through shearing, stamping orsaw cutting.

Example 3

Example 1is modified in that 119.5 parts by weight tin oxide powder withparticle size smaller than 7 mm and 0.5 parts by weight Ag₂ MoO₄ with amedium particle size of 40 mm are mixed and annealed at 600 deg. C. Inthis process Ag₂ MoO₄ is spread to the tin oxide particles. Theremainder of the procedure is the same as in example 1.

Contact pieces produced in this manner are tested for life spanaccording to test category AC1in a switching equipment having poweroutput of 37 kW. After 200,000 switching operations the life time testwas interrupted for a check on the temperature rise of the contactpieces under continuous current. It could be shown that the temperaturerise with 70 to 90 deg. K in the average was not higher than for aconventionally produced material of composition Ag88/SnO₂ 11.6/MoO₃ 0.4containing approximately 10 times as much molybdenum-oxide.

The three examples can be modified in that tin oxide is replaced byzinc-oxide.

We claim:
 1. A process for the production of a material for electriccontacts on the basis of silver-tin oxide comprising the steps of:mixingtin oxide powder and an additional powder, the weight % of theadditional powder is 0.01 to 10 weight % of the weight % of tin oxide,said additional powder comprises at least one compound consisting ofsilver, oxygen and a metal selected from the group consisting ofSub-groups II to VI of the Periodic Table, antimony, bismuth, germanium,indium and gallium; annealing the mixture; mixing said annealed mixturewith a silver or silver based alloy powder forming a mixture comprising5-20 weight % tin oxide, the weight % of the additional powder is0.01-10 weight % of the weight % of tin oxide, and the remaining weight% of the mixture comprises silver or silver based alloy powder;compacting the mixture; and sintering the mixture.
 2. The processaccording to claim 1, wherein the mixture is annealed in air.
 3. Theprocess according to claim 1, wherein the mixture is annealed at atemperature range between 500 degrees C. and 800 degrees C.
 4. Theprocess according to claim 1, wherein the mixture is annealed at atemperature above the melting temperature of the additional powder. 5.The process according to claim 1, wherein the material is compactedagain after sintering and further re-shaped.
 6. A process for theproduction of a material for electric contacts on the basis of zincoxide comprising the steps of:mixing zinc oxide powder and an additionalpowder, the weight % of the additional powder is 0.01 to 10 weight % ofthe weight % of zinc oxide, said additional powder comprises at leastone compound consisting of silver, oxygen and a metal selected from thegroup consisting of Sub-groups II to VI of the Periodic Table, antimony,bismuth, germanium, indium and gallium: annealing the mixture; mixingsaid annealed mixture with a silver or silver based alloy powder forminga mixture comprising 5-20 weight % zinc oxide, the weight % of theadditional powder is 0.01-10 weight % of the weight % of zinc oxide, andthe remaining weight % of the mixture comprises silver or silver basedalloy powder; compacting the mixture; and sintering the mixture.
 7. Theprocess according to claim 6, wherein the mixture is annealed in air. 8.The process according to claim 6, wherein the mixture is annealed at atemperature range between 500 degrees C. and 800 degrees C.
 9. Theprocess according to claim 6, wherein the mixture is annealed at atemperature above the melting temperature of the additional powder. 10.The process according to claim 6, wherein after the sintering mixture iscompacted again and further re-shaped.